1. Field of the Invention
The present invention relates to a glass panel used in image display devices (e.g. a cathode ray tube, plasma display panel) or fluorescent indicator tubes which are irradiated with electron rays, particularly to a glass panel suitably used in cathode ray tubes which are irradiated with electron rays of high energy density.
2. Description of Related Art
Currently, display devices using a cathode ray tube or a plasma display are required to use electron rays of high acceleration voltage or high energy density in order to increase the luminance, or to decrease the thickness, of the glass panel in order to reduce the total weight.
Hence, the glass panels used in said display devices are required to (a) undergo low coloring when irradiated with electron rays, (b) have a high strength even when the panels are thin and (c) be resistant to dielectric breakdown even when a high voltage is applied thereto. Further, it is necessary that the glass ingredients be sufficiently available in order to enable production, of glass panels meeting the above requirements in a large amount. For these reasons, studies have been under way on various glass compositions containing no PbO (PbO increases the weight of resulting glass). That is, it is under study to heat-form a glass plate material containing no PbO, then substitute the Na ion present at the plate surface with K ion to generate a compression layer at the plate surface and to thereby produce a glass panel having a higher mechanical strength while incurring low coloring when irradiated with electron rays. In such a glass panel, the glass composition, particularly the Na.sub.2 O, K.sub.2 O and Li.sub.2 O contents and the viscosity characteristic of glass plate material are important. Japanese Patent Application Kokai (Laid-open) No. 288134/1987 discloses a glass panel containing SrO and BaO and having a controlled ratio of Na.sub.2 O and K.sub.2 O contents. This glass panel has a high K.sub.2 O content and therefore has a good electrical insulating property; however, even after being subjected to an ion exchange treatment (for substitution of Na ion in panel with K ion) in a molten salt containing K ion, the panel is unable to have a higher mechanical strength and shows insufficient suppression for coloring caused by irradiation with electron rays. Further, the glass plate material used for the production of the above glass panel cannot be produced by shaping a molten glass material directly into a flat plate according to the float process, and must be produced by subjecting a glass gob to press molding, followed by surface grinding.
Meanwhile, Japanese Patent Application Kokai (Laid-open) No. 153143/1987 discloses a glass plate material which can be produced as a flat plate directly by the float process. The glass panel produced therefrom has a high electrical insulating property but, even after being subjected to an ion exchange treatment, is unable to show improvements in resistance to coloring caused by irradiation with electron rays as well as in mechanical strength.
Further, Japanese Patent Application Kokai (Laid-open) No. 195146/1988 discloses a glass panel which is produced by subjecting a glass plate material of soda-lime silica composition for commercially available window glass, obtained by the float process, to an ion exchange treatment in a K-containing molten salt to form a K-ion-rich layer at the panel surface and which resultantly has sufficient resistance to coloring when irradiated with electron rays and sufficient mechanical strength. The glass panel, however, has a low electrical insulating property and, when receiving a high voltage locally, gives rise to dielectric breakdown.
The present invention has been made in order to solve the above-mentioned problems of the prior art. The object of the present invention is to provide a glass panel characterized by (a) a suppression of the coloring caused by electron rays irradiation, (b) a high mechanical strength owing to the presence of a compression stress layer formed at the panel surface by an ion exchange treatment, and (c) a high electrical insulating property and therefore being resistant to dielectric breakdown even when a high voltage is applied to the panel, and further characterized in that the glass plate material used for production of said glass panel can be produced directly from a molten glass material by the float process.